The incidence of spontaneous defibrillation following open heart surgery was found to be 38% and correlated with serum potassium which was higher in spontaneous defibrillation, time of aortic unclamping, which was shorter in spontaneously defibrillating patients, and left ventricular wall thickness, which was less in spontaneously defibrillating patients. In patients requiring defibrillation, the energy requirements varied from 2.8 to 10 joules delivered energy, current levels from 2.3 to 13.1 amps, and the resistance of the heart was 21-22 ohms. Factors significant for successful shocks were a longer period of reperfusion (longer aortic declamping to defibrillation time), a higher serum potassium, a higher septal temperature and a higher energy level. Using discriminant analysis, comparing all successful shock against all failed shocks, failed shocks were more likely to have pump failure postoperatively and fine ventricular fibrillation, while a higher septal temperature correlated with a successful shock.